Method and apparatus for acquiring system information in wireless communication system

ABSTRACT

A method and apparatus for acquiring system information in a wireless communication system are disclosed. The method for acquiring system information by a user equipment (UE) in a wireless communication system includes: performing a handover from a first cell to a second cell; and acquiring system information of the second cell, wherein a method for acquiring the system information of the second cell is determined according to CRE (Cell Range Extension)-related capability of the UE.

TECHNICAL FIELD

The present invention relates to a wireless communication system, andmore particularly to a method and apparatus for acquiring systeminformation in a wireless communication system.

BACKGROUND ART

Wireless communication systems have been widely used to provide variouskinds of communication services such as voice or data services.Generally, a wireless communication system is a multiple access systemthat can communicate with multiple users by sharing available systemresources (bandwidth, transmission (Tx) power, and the like). A varietyof multiple access systems can be used. For example, a Code DivisionMultiple Access (CDMA) system, a Frequency Division Multiple Access(FDMA) system, a Time Division Multiple Access (TDMA) system, anOrthogonal Frequency Division Multiple Access (OFDMA) system, a SingleCarrier Frequency-Division Multiple Access (SC-FDMA) system, aMulti-Carrier Frequency Division Multiple Access (MC-FDMA) system, andthe like.

DISCLOSURE Technical Problem

Accordingly, the present invention is directed to a method and apparatusfor acquiring system information in a wireless communication system thatsubstantially obviates one or more problems due to limitations anddisadvantages of the related art.

An object of the present invention is to provide technologies related tosystem information acquisition in a serious interference situation.

It is to be understood that technical objects to be achieved by thepresent invention are not limited to the aforementioned technicalobjects and other technical objects which are not mentioned herein willbe apparent from the following description to one of ordinary skill inthe art to which the present invention pertains.

Technical Solution

The object of the present invention can be achieved by providing amethod for acquiring system information by a user equipment (UE) in awireless communication system including: performing a handover from afirst cell to a second cell; and acquiring system information of thesecond cell, wherein a method for acquiring the system information ofthe second cell is determined according to CRE (Cell RangeExtension)-related capability of the UE.

In another aspect of the present invention, a user equipment (UE)apparatus for use in a wireless communication system includes: areception (Rx) module, and a processor, wherein the processor performs ahandover from a first cell to a second cell and acquires systeminformation of the second cell, wherein a method for acquiring thesystem information of the second cell is determined according to CRE(Cell Range Extension)-related capability of the UE.

If the UE has PBCH (Physical Broadcast Channel) cancellation capability,the UE may acquire system information of the second cell through a PBCHof the second cell.

The UE may perform PBCH cancellation of the first cell when acquiringthe PBCH of the second cell.

If the UE does not have the PBCH cancellation capability, the UE mayacquire system information of the second cell from the first cell.

The system information of the second cell may be transferred to the UEin a step of performing the handover.

The method may further include: if a predetermined condition issatisfied after completion of the handover, transmitting an error reportto the second cell.

The predetermined condition may that the UE fail to successively receivesignals from the second cell.

The method may further include: receiving at least one of changed systeminformation and specific information indicating no change of the systeminformation, as a response to the error report.

The method may further include: if the UE is located within apredetermined distance of the second cell after completion of thehandover, transmitting a request of system information to the secondcell.

The method may further include: receiving system information masked withC-RNTI of the UE, as a response to the system information request.

The system information request may be any one of an MIB (MasterInformation Block) request, an SIB (System Information Block) request,and a paging request, which are updated information of the second cell.

The first cell may be a macro eNB, and a second cell may be a pico eNB.

The CRE-related capability information may include at least one ofreference signal (RS) cancellation capability and synchronization signalcancellation capability.

Advantageous Effects

As is apparent from the above description, exemplary embodiments of thepresent invention can stably receive system information even in aserious interference situation.

It will be appreciated by persons skilled in the art that the effectsthat can be achieved with the present invention are not limited to whathas been particularly described hereinabove and other advantages of thepresent invention will be more clearly understood from the followingdetailed description taken in conjunction with the accompanyingdrawings.

DESCRIPTION OF DRAWINGS

The accompanying drawings, which are included to provide a furtherunderstanding of the invention, illustrate embodiments of the inventionand together with the description serve to explain the principle of theinvention.

FIG. 1 exemplarily shows a radio frame structure.

FIG. 2 exemplarily shows a resource grid of a downlink slot.

FIG. 3 exemplarily shows a downlink subframe structure.

FIG. 4 exemplarily shows an uplink subframe structure.

FIG. 5 is a conceptual diagram illustrating a reference signal (RS).

FIG. 6 is a conceptual diagram illustrating a cooperative transmissioncluster.

FIG. 7 is a conceptual diagram illustrating CoMP (Coordinated MultiPoint) cluster.

FIG. 8 is a conceptual diagram illustrating restricted measurement.

FIG. 9 is a conceptual diagram illustrating Cell Ranging Expansion(CRE).

FIG. 10 is a conceptual diagram illustrating summaries of embodiments ofthe present invention.

FIGS. 11 to 13 are flowcharts illustrating methods for measuring aneighbor cell according to embodiments of the present invention.

FIG. 14 is a flowchart illustrating a method for measuring a servingcell according to embodiments of the present invention.

FIG. 15 is a block diagram illustrating a transceiver apparatusapplicable to embodiments of the present invention.

BEST MODE

The following embodiments are proposed by combining constituentcomponents and characteristics of the present invention according to apredetermined format. The individual constituent components orcharacteristics should be considered optional factors on the conditionthat there is no additional remark. If required, the individualconstituent components or characteristics may not be combined with othercomponents or characteristics. Also, some constituent components and/orcharacteristics may be combined to implement the embodiments of thepresent invention. The order of operations to be disclosed in theembodiments of the present invention may be changed. Some components orcharacteristics of any embodiment may also be included in otherembodiments, or may be replaced with those of the other embodiments asnecessary.

The embodiments of the present invention are disclosed on the basis of adata communication relationship between a base station and a terminal.In this case, the base station is used as a terminal node of a networkvia which the base station can directly communicate with the terminal.Specific operations to be conducted by the base station in the presentinvention may also be conducted by an upper node of the base station asnecessary.

In other words, it will be obvious to those skilled in the art thatvarious operations for enabling the base station to communicate with theterminal in a network composed of several network nodes including thebase station will be conducted by the base station or other networknodes other than the base station. The term “Base Station (BS)” may bereplaced with a fixed station, Node-B, eNode-B (eNB), or an access pointas necessary. The term “relay” may be replaced with the terms Relay Node(RN) or Relay Station (RS). The term “terminal” may also be replacedwith a User Equipment (UE), a Mobile Station (MS), a Mobile SubscriberStation (MSS) or a Subscriber Station (SS) as necessary.

It should be noted that specific terms disclosed in the presentinvention are proposed for convenience of description and betterunderstanding of the present invention, and the use of these specificterms may be changed to other formats within the technical scope orspirit of the present invention.

In some instances, well-known structures and devices are omitted inorder to avoid obscuring the concepts of the present invention andimportant functions of the structures and devices are shown in blockdiagram form. The same reference numbers will be used throughout thedrawings to refer to the same or like parts.

Exemplary embodiments of the present invention are supported by standarddocuments disclosed for at least one of wireless access systemsincluding an Institute of Electrical and Electronics Engineers (IEEE)802 system, a 3^(rd) Generation Partnership Project (3GPP) system, a3GPP Long Term Evolution (LTE) system, an LTE-Advanced (LTE-A) system,and a 3GPP2 system. In particular, steps or parts, which are notdescribed to clearly reveal the technical idea of the present invention,in the embodiments of the present invention may be supported by theabove documents. All terminology used herein may be supported by atleast one of the above-mentioned documents.

The following embodiments of the present invention can be applied to avariety of wireless access technologies, for example, CDMA (CodeDivision Multiple Access), FDMA (Frequency Division Multiple Access),TDMA (Time Division Multiple Access), OFDMA (Orthogonal FrequencyDivision Multiple Access), SC-FDMA (Single Carrier Frequency DivisionMultiple Access), and the like. CDMA may be embodied through wireless(or radio) technology such as UTRA (Universal Terrestrial Radio Access)or CDMA2000. TDMA may be embodied through wireless (or radio) technologysuch as GSM (Global System for Mobile communication)/GPRS (GeneralPacket Radio Service)/EDGE (Enhanced Data Rates for GSM Evolution).OFDMA may be embodied through wireless (or radio) technology such asInstitute of Electrical and Electronics Engineers (IEEE) 802.11 (Wi-Fi),IEEE 802.16 (WiMAX), IEEE 802-20, and E-UTRA (Evolved UTRA). UTRA is apart of UMTS (Universal Mobile Telecommunications System). 3GPP (3rdGeneration Partnership Project) LTE (long term evolution) is a part ofE-UMTS (Evolved UMTS), which uses E-UTRA. 3GPP LTE employs OFDMA indownlink and employs SC-FDMA in uplink. LTE-Advanced (LTE-A) is anevolved version of 3GPP LTE. WiMAX can be explained by IEEE 802.16e(WirelessMAN-OFDMA Reference System) and advanced IEEE 802.16m(WirelessMAN-OFDMA Advanced System). For clarity, the followingdescription focuses on IEEE 802.11 systems. However, technical featuresof the present invention are not limited thereto.

LTE/LTE-A Resource Structure/Channel

FIG. 1 exemplarily shows a radio frame structure.

The structure of a radio frame in 3GPP LTE system will be described withreference to FIG. 1. In a cellular Orthogonal Frequency DivisionMultiplexing (OFDM) radio packet communication system, uplink/downlinkdata packet transmission is performed in subframe units. One subframe isdefined as a predetermined time interval including a plurality of OFDMsymbols. The 3GPP LTE standard supports a type 1 radio frame structureapplicable to Frequency Division Duplex (FDD) and a type 2 radio framestructure applicable to Time Division Duplex (TDD).

FIG. 1( a) is a diagram showing the structure of the type 1 radio frame.A radio frame includes 10 subframes, and one subframe includes two slotsin the time domain. A time required for transmitting one subframe isdefined in a Transmission Time Interval (TTI). For example, one subframemay have a length of 1 ms and one slot may have a length of 0.5 ms. Oneslot may include a plurality of OFDM symbols in time domain and includea plurality of Resource Blocks (RBs) in frequency domain. Since the 3GPPLTE system uses OFDMA in downlink, the OFDM symbol indicates one symbolduration. The OFDM symbol may be called an SC-FDMA symbol or a symbolduration. An RB is a resource allocation unit and includes a pluralityof contiguous subcarriers in one slot.

The number of OFDM symbols included in one slot may be changed accordingto the configuration of a Cyclic Prefix (CP). The CP includes anextended CP and a normal CP. For example, if the OFDM symbols areconfigured by the normal CP, the number of OFDM symbols included in oneslot may be seven. If the OFDM symbols are configured by the extendedCP, the length of one OFDM symbol is increased, the number of OFDMsymbols included in one slot is less than that of the case of the normalCP. In case of the extended CP, for example, the number of OFDM symbolsincluded in one slot may be six. If a channel state is unstable, forexample, if a User Equipment (UE) moves at a high speed, the extended CPmay be used in order to further reduce interference between symbols.

In case of using the normal CP, since one slot includes seven OFDMsymbols, one subframe includes 14 OFDM symbols. At this time, the firsttwo or three OFDM symbols of each subframe may be allocated to aPhysical Downlink Control Channel (PDCCH) and the remaining OFDM symbolsmay be allocated to a Physical Downlink Shared Channel (PDSCH).

The structure of a type 2 radio frame is shown in FIG. 1( b). The type 2radio frame includes two half-frames, each of which is made up of fivesubframes, a downlink pilot time slot (DwPTS), a guard period (GP), andan uplink pilot time slot (UpPTS), in which one subframe consists of twoslots. That is, one subframe is composed of two slots irrespective ofthe radio frame type. DwPTS is used to perform initial cell search,synchronization, or channel estimation. UpPTS is used to perform channelestimation of a base station and uplink transmission synchronization ofa user equipment (UE). The guard interval (GP) is located between anuplink and a downlink so as to remove interference generated in theuplink due to multi-path delay of a downlink signal. That is, onesubframe is composed of two slots irrespective of the radio frame type.

The structure of the radio frame is only exemplary. Accordingly, thenumber of subframes included in the radio frame, the number of slotsincluded in the subframe or the number of symbols included in the slotmay be changed in various manners.

FIG. 2 is a diagram showing a resource grid in a downlink slot. Althoughone downlink slot includes seven OFDM symbols in a time domain and oneRB includes 12 subcarriers in a frequency domain in the figure, thescope or spirit of the present invention is not limited thereto. Forexample, in case of a normal Cyclic Prefix (CP), one slot includes 7OFDM symbols. However, in case of an extended CP, one slot may include 6OFDM symbols. Each element on the resource grid is referred to as aresource element. One RB includes 12×7 resource elements. The numberN^(DL) of RBs included in the downlink slot is determined based ondownlink transmission bandwidth. The structure of the uplink slot may beequal to the structure of the downlink slot.

FIG. 3 is a diagram showing the structure of a downlink subframe. Amaximum of three OFDM symbols of a front portion of a first slot withinone subframe corresponds to a control region to which a control channelis allocated. The remaining OFDM symbols correspond to a data region towhich a Physical Downlink Shared Channel (PDSCH) is allocated. The basicunit of transmission becomes one subframe. Examples of the downlinkcontrol channels used in the 3GPP LTE system include, for example, aPhysical Control Format Indicator Channel (PCFICH), a Physical DownlinkControl Channel (PDCCH), a Physical Hybrid automatic repeat requestIndicator Channel (PHICH), etc. The PCFICH is transmitted at a firstOFDM symbol of a subframe, and includes information about the number ofOFDM symbols used to transmit the control channel in the subframe. ThePHICH includes a HARQ ACK/NACK signal as a response to uplinktransmission. The control information transmitted through the PDCCH isreferred to as Downlink Control Information (DCI). The DCI includesuplink or downlink scheduling information or an uplink transmit powercontrol command for a certain UE group. The PDCCH may include resourceallocation and transmission format of a Downlink Shared Channel(DL-SCH), resource allocation information of an Uplink Shared Channel(UL-SCH), paging information of a Paging Channel (PCH), systeminformation on the DL-SCH, resource allocation of a higher layer controlmessage such as a Random Access Response (RAR) transmitted on the PDSCH,a set of transmit power control commands for individual UEs in a certainUE group, transmit power control information, activation of Voice overIP (VoIP), etc. A plurality of PDCCHs may be transmitted within thecontrol region. The UE may monitor the plurality of PDCCHs. The PDCCHsare transmitted as an aggregate of one or several contiguous controlchannel elements (CCEs). The CCE is a logical allocation unit used toprovide the PDCCHs at a coding rate based on the state of a radiochannel. The CCE corresponds to a plurality of resource element groups.The format of the PDCCH and the number of available bits are determinedbased on a correlation between the number of CCEs and the coding rateprovided by the CCEs. The eNB (or base station) determines a PDCCHformat according to a DCI to be transmitted to the UE, and attaches aCyclic Redundancy Check (CRC) to control information. The CRC is maskedwith a Radio Network Temporary Identifier (RNTI) according to an owneror usage of the PDCCH. If the PDCCH is for a specific UE, a cell-RNTI(C-RNTI) of the UE may be masked to the CRC. Alternatively, if the PDCCHis for a paging message, a paging indicator identifier P-RNTI) may bemasked to the CRC. If the PDCCH is for system information (morespecifically, a system information block (SIB)), a system informationidentifier and a system information RNTI (SI-RNTI) may be masked to theCRC. To indicate a random access response that is a response fortransmission of a random access preamble of the UE, a random access-RNTI(RA-RNTI) may be masked to the CRC.

FIG. 4 is a diagram showing the structure of an uplink frame. The uplinksubframe may be divided into a control region and a data region in afrequency domain. A Physical Uplink Control Channel (PUCCH) includinguplink control information is allocated to the control region. APhysical Uplink Shared Channel (PUSCH) including user data is allocatedto the data region. In order to maintain single carrier characteristics,one UE does not simultaneously transmit the PUCCH and the PUSCH. ThePUCCH for one UE is allocated to an RB pair in a subframe. RBs belongingto the RB pair occupy different subcarriers with respect to two slots.Thus, the RB pair allocated to the PUCCH is “frequency-hopped” at a slotedge.

Reference Signal (RS)

In a wireless communication system, since packets are transmittedthrough a radio channel, a signal may be distorted during transmission.In order to enable a reception side to correctly receive the distortedsignal, distortion of the received signal should be corrected usingchannel information. In order to detect the channel information, amethod of transmitting a signal, of which both the transmission side andthe reception side are aware, and detecting channel information using adistortion degree when the signal is received through a channel ismainly used. The above signal is referred to as a pilot signal or areference signal (RS).

When transmitting and receiving data using multiple antennas, thechannel states between the transmission antennas and the receptionantennas should be detected in order to correctly receive the signal.Accordingly, each transmission antenna has an individual RS. In moredetail, an independent RS should be transmitted through each Tx port.

RS may be divided into downlink RS and uplink RS. In the current LTEsystem, the uplink RS include:

i) DeModulation-Reference Signal (DM-RS) used for channel estimation forcoherent demodulation of information delivered on a PUSCH and a PUCCH;and

ii) Sounding Reference Signal (SRS) used for a BS (eNB) or a network tomeasure the quality of an uplink channel in a different frequency.

The downlink RS are categorized into:

i) Cell-specific Reference Signal (CRS) shared among all UEs of a cell;

ii) UE-specific RS dedicated to a specific UE;

iii) DM-RS used for coherent demodulation of a PDSCH, when the PDSCH istransmitted;

iv) Channel State Information-Reference Signal (CSI-RS) carrying CSI,when downlink DM-RS are transmitted;

v) Multimedia Broadcast Single Frequency Network (MBSFN) RS used forcoherent demodulation of a signal transmitted in MBSFN mode; and

vi) positioning RS used to estimate geographical position informationabout a UE.

RS may also be divided into two types according to their purposes: RSfor channel information acquisition and RS for data demodulation. Sinceits purpose lies in that a UE acquires downlink channel information, theformer should be transmitted in a broad band and received even by a UEthat does not receive downlink data in a specific subframe. This RS isalso used in a situation like handover. The latter is an RS that a BS(eNB) transmits along with downlink data in specific resources. A UE candemodulate the data by measuring a channel using the RS. This RS shouldbe transmitted in a data transmission area.

CRS serve two purposes, that is, channel information acquisition anddata demodulation. A UE-specific RS is used only for data demodulation.CRS are transmitted in every subframe in a broad band and CRS for up tofour antenna ports are transmitted according to the number of Txantennas in an eNB.

For example, if the BS (eNB) has two Tx antennas, CRS for antenna ports0 and 1 are transmitted. In the case of four Tx antennas, CRS forantenna ports 0 to 3 are respectively transmitted.

FIG. 5 illustrates patterns in which CRS and DRS are mapped to adownlink RB pair, as defined in a legacy 3GPP LTE system (e.g.Release-8). An RS mapping unit, i.e. a downlink RB pair may include onesubframe in time by 12 subcarriers in frequency. That is, an RB pairincludes 14 OFDM symbols in time in the case of the normal CP (see FIG.5( a)) and 12 OFDM symbols in time in the case of the extended CP (seeFIG. 5( b)).

In FIG. 5, the positions of RS in an RB pair for a system where a BS(eNB) supports four Tx antennas are illustrated. Reference numerals 0,1, 2 and 3 denote the REs of CRS for first to fourth antenna ports,antenna port 0 to antenna port 3, respectively, and reference character‘D’ denotes the positions of DRS.

CSI (Channel Status Information) Feedback

MIMO schemes are classified into an open-loop MIMO scheme and aclosed-loop MIMO scheme. The open-loop MIMO scheme means that atransmitter performs MIMO transmission without receiving CSI feedbackinformation from a MIMO receiver. The closed-loop MIMO scheme means thata transmitter receives CSI feedback information from the MIMO receiverand performs MIMO transmission. In accordance with the closed-loop MIMOscheme, each of a transmitter and a receiver can perform beamforming onthe basis of CSI so as to obtain a multiplexing gain of a MIMOtransmission antenna. The transmitter (for example, BS) can allocate anuplink control channel or an uplink shared channel to a receiver (forexample, a user equipment) in such a manner that the receiver can feedback the CSI.

The feedback CSI may include a rank indicator (RI), a precoding matrixindex (PMI), and a channel quality indicator (CQI).

RI is information of a channel rank. The channel rank means a maximumnumber of layers (or streams) via which different information can betransmitted through the same time-frequency resources. Since a rankvalue is determined depending on long-term fading of a channel, the rankvalue is generally fed back for a longer period than PMI and CQI. Thatis, the rank value can be fed back less frequently than PMI and CQI.

PMI is information regarding a precoding matrix used for datatransmission from the transmitter, and includes spatial characteristicsof a channel. Precoding means that a transmit layer is mapped to atransmit antenna, and the layer-antenna mapping relationship can bedetermined by precoding matrices. PMI corresponds to a UE-preferredprecoding matrix index of a BS on the basis of metric data such asSignal-to-Interference plus Noise Ratio (SINR). In order to reducefeedback overhead of the precoding information, a transmitter and areceiver may share a variety of precoding matrices in advance, and onlyindices indicating a specific precoding matrix from among thecorresponding codebook can be fed back.

Acquisition of an additional multi-user diversity using Multi-User MIMO(MU-MIMO) is under consideration in a system supporting an extendedantenna configuration (e.g. an LTE-A system). In MU-MIMO, aninterference channel exists between UEs multiplexed in an antennadomain. Therefore, when the eNB transmits a downlink signal based on CSIfeedback received from one UE, it needs to perform the downlinktransmission in a manner that avoids interference with other UEs. Hence,for a reliable MU-MIMO operation, CSI should be fed back with moreaccuracy than for a Single-User MIMO (SU-MIMO) operation.

To enable more accurate CSI measurement and reporting, a new CSIfeedback scheme may be used by improving conventional CSI including anRI, a PMI, and a CQI. For example, precoding information fed back by areceiver may be indicated by a combination of two PMIs. One of the twoPMIs (a first PMI) may be referred to as W1 having a long term and/orwideband property and the other PMI (a second PMI) may be referred to asW1 having a short term and/or subband property. A final PMI may bedetermined by a function of W1 and W2. For example, let the final PMI bedenoted by W. Then it may defined that W=W1*W2 or W=W2*W1.

CQI is information indicating channel quality or channel strength. CQImay be represented by a combination of predetermined MCSs. That is, afeedback CQI index may indicate a modulation scheme and a code rate.Generally, a reception SINR capable of being obtained when the BSconstructs a spatial channel using a PMI is applied to CQI.

The current LTE/LTE-A system defines ‘CSI reference resource’ related tochannel measurement for the above-described CSI feedback/reporting. TheCSI reference resource is defined by a group of physical RBscorresponding to a frequency band for which a CQI is calculated in thefrequency domain. From a time perspective, for CSItransmission/reporting in subframe n, the CSI reference resource isdefined by a single downlink subframe, n-n_(CQI) _(—) _(ref). i) Forperiodic CSI reporting, n-n_(CQI) _(—) _(ref) is the smallest valuegreater than or equal to 4, which is a valid downlink subframe. ii) Foraperiodic CSI reporting, n-n_(CQI) _(—) _(ref) is typically thereference resource in the same valid downlink subframe as acorresponding CSI request in a DCI format for uplink transmissions. iii)For aperiodic CSI reporting triggered by a Random Access Response Grantcarrying a CSI request, n-n_(CQI) _(—) _(ref) equals 4. A downlinksubframe is considered valid when it is configured as a downlinksubframe for a particular UE, it is not an MBSFN subframe except forMode 9, it contains a DwPTS with a predetermined size or larger, it doesnot fall within a configured measurement gap for that UE, and forperiodic CSI reporting, it should be an element of a CSI subframe setwhen that UE is configured with CSI subframe sets. A higher layer mayconfigure CSI subframe sets (C_(CSI0), C_(CSI,1)) for the UE. Thecurrent standard defines that the CSI reference resource is included inone of the two CSI subframe sets (C_(CSI,0), C_(CSI,1)), not in both.

Heterogeneous Deployments

FIG. 6 is a heterogeneous network wireless communication systemincluding a macro eNB (MeNB) and a micro eNB (PeNB or FeNB). The term“heterogeneous network” refers to a network in which a macro eNB (MeNB)and a micro eNB (PeNB or FeNB) coexist even though the same Radio AccessTechnology (RAT) is used.

The macro eNB (MeNB) is a normal eNB having wide coverage and hightransmission power in a wireless communication system. The macro eNB(MeNB) may also be referred to as a macro cell.

The micro eNB (PeNB or FeNB) may also be referred to as a micro cell, apico cell, a femto cell, a home eNB (HeNB), a relay, etc. (MeNB, PeNBand FeNB may also be generically named a transmission point asnecessary). The micro eNB (PeNB or FeNB) is a small-sized version of themacro eNB (MeNB), such that the micro eNB (PeNB or FeNB) mayindependently perform most of the functions of the macro eNB (MeNB). Themicro eNB (PeNB or FeNB) may be installed (in an overlay manner) in anarea covered by the macro eNB (MeNB) or may be installed (in anon-overlay manner) in a shadow area that cannot be covered by the macroeNB (MeNB). The micro eNB (PeNB or FeNB) has a narrower coverage andlower transmission power and may accommodate a smaller number of userequipments (UEs), compared to the micro eNB (MeNB).

UE, which is hereinafter referred to as a macro UE (MUE), may bedirectly served by the macro eNB (MeNB). UE, which is hereinafterreferred to as a micro UE (MUE), may be served by the micro eNB (PeNB orFeNB). In some cases, the UE present within the coverage of the microeNB (MeNB) may be served by the macro eNB (MeNB).

The micro eNB (eNB) may be classified into two types according to accesslimitations of the UE.

The first type is a Closed Subscriber Group (CSG) or non-Closed AccessSubscriber Group (non-CSG) eNB serving as a cell that allows either alegacy macro UE or another micro eNB to access a micro UE. The legacymacro UE (MUE) or the like may be handed over to an OSG-type eNB.

The second type is a CSG eNB that prevents the legacy macro UE oranother micro eNB from accessing the micro UE, such that it isimpossible to be handed over to the CSG eNB.

Coordinated Multi-Point (CoMP)

According to the improved system performance requirements of the 3GPPLTE-A system, CoMP transmission/reception technology (may be referred toas co-MIMO, collaborative MIMO or network MIMO) is proposed. The CoMPtechnology can increase the performance of the UE located on a cell edgeand increase average sector throughput.

In general, in a multi-cell environment in which a frequency reusefactor is 1, the performance of the UE located on the cell edge andaverage sector throughput may be reduced due to Inter-Cell Interference(ICI). In order to reduce the ICI, in the existing LTE system, a methodof enabling the UE located on the cell edge to have appropriatethroughput and performance using a simple passive method such asFractional Frequency Reuse (FFR) through the UE-specific power controlin the environment restricted by interference is applied. However,rather than decreasing the use of frequency resources per cell, it ispreferable that the ICI is reduced or the UE reuses the ICI as a desiredsignal. In order to accomplish the above object, a CoMP transmissionscheme may be applied.

The CoMP scheme applicable to the downlink may be largely classifiedinto a Joint Processing (JP) scheme and a CoordinatedScheduling/Beamforming (CS/CB) scheme.

In the JP scheme, each point (eNodeB) of a CoMP unit may use data. TheCoMP unit refers to a set of eNodeBs used in the CoMP scheme. The JPscheme may be classified into a joint transmission scheme and a dynamiccell selection scheme.

The joint transmission scheme refers to a scheme for transmitting aPDSCH from a plurality of points (a part or the whole of the CoMP unit).That is, data transmitted to a single UE may be simultaneouslytransmitted from a plurality of transmission points. According to thejoint transmission scheme, it is possible to coherently ornon-coherently improve the quality of the received signals and toactively eliminate interference with another UE.

The dynamic cell selection scheme refers to a scheme for transmitting aPDSCH from one point (of the CoMP unit). That is, data transmitted to asingle UE at a specific time is transmitted from one point and the otherpoints in the cooperative unit at that time do not transmit data to theUE. The point for transmitting the data to the UE may be dynamicallyselected.

According to the CS/CB scheme, the CoMP units may cooperatively performbeamforming of data transmission to a single UE. Although only a servingcell transmits the data, user scheduling/beamforming may be determinedby the coordination of the cells of the CoMP unit.

In uplink, coordinated multi-point reception refers to reception of asignal transmitted by coordination of a plurality of geographicallyseparated points. The CoMP scheme applicable to the uplink may beclassified into Joint Reception (JR) and CoordinatedScheduling/Beamforming (CS/CB).

The JR scheme indicates that a plurality of reception points receives asignal transmitted through a PUSCH, the CS/CB scheme indicates that onlyone point receives a PUSCH, and user scheduling/beamforming isdetermined by the coordination of the cells of the CoMP unit.

In this CoMP system, multi-cell BSs (eNBs) can support data for a UE. Inaddition, the BSs (eNBs) support one or more UEs simultaneously in thesame radio frequency resources, thereby increasing system performance.The BSs (eNBs) may also operate in Space Division Multiple Access (SDMA)based on CSI between a UE and the eNBs.

A serving BS (eNB) and one or more cooperative BSs (eNBs) are connectedto a scheduler through a backbone network in the CoMP system. Thescheduler may receive channel information about the channel statesbetween a UE and the cooperative eNBs, measured by each cooperative BS(eNB) and operate based on the channel information. For example, thescheduler may schedule information for cooperative MIMO for the servingBS (eNB) and the one or more cooperative BSs (eNBs). That is, thescheduler may transmit a command directly to each eNB in regard to thecooperative MIMO operation.

As can be seen from the above description, it can be recognized that aCoMP system operates as a virtual MIMO system by grouping a plurality ofcells into one group. Basically, the CoMP system adopts a MIMOcommunication scheme using multiple antennas.

A CoMP cluster is a set of cells that are capable of performing the CoMPoperations (i.e., cooperative scheduling and cooperative datatransmission/reception). For example, cells of a single cluster may beassigned different physical cell IDs (PCIDs) as shown in FIG. 7( a), andcells of a single cluster may share the same PCIDs such that the cellsmay be configured in the form of a distributed antenna or RRH of asingle BS. In modified examples of FIG. 7, some cells from among cellsof the single cluster may share the same PCIDs.

Generally, cells of the same CoMP cluster are interconnected through abackhaul link, such as an optical fiber having high capacity and lowlatency, so as to implement cooperative scheduling and cooperative datatransmission/reception, such that the cooperative scheduling is possibleand maintained at a correct time synchronization state, resulting inimplementation of cooperative data transmission. In addition, whenreceiving signals from cells of the CoMP cluster participating in thecooperative transmission, the size of CoMP cluster must be determined ina manner that a reception time difference between signals transmittedfrom respective cells can enter the scope of a cyclic prefix (CP) lengthon the basis of a propagation delay difference between respective cells.In contrast, cells belonging to different clusters may be interconnectedthrough a lower-capacity backhaul link, and may not maintain timesynchronization.

A UE configured to perform CoMP can perform cooperative scheduling andcooperative data transmission/reception by some or all of cellscontained in the CoMP cluster, and the UE measures a reference signalthat is transmitted from some or all cells of the CoMP cluster accordingto a UE reception signal quality (i.e., QoS of a UE reception signal).In order to measure link performances of UE and each cell, the UE maymeasure a reference signal of each cell and may report a QoS of themeasured reference signal. Specifically, cells to be measured by the UEmay be defined as a CoMP measurement set.

For CoMP, there is a need to define the reference resource set throughwhich UE channel measurement and UE channel measurement reporting mustbe performed, because the CoMP scheme and downlink scheduling, etc. ofthe corresponding UE are determined according to per-cell channelinformation to be reported by the UE on uplink. Information (i.e., theCoMP measurement set) indicating that the UE must measure/report signalsfrom a certain cell should be transferred through higher layersignaling, and associated information can be signaled as CSI-RSresources.

Inter-Cell Interference Coordination (ICIC)

In the above-mentioned heterogeneous network environment (heterogeneousdeployment) or CoMP environment, inter-cell interference (ICI) mayoccur. In order to solve the inter-cell interference (ICI) problem, aninter-cell interference coordination (ICIC) may be used.

As an exemplary ICIC of the frequency resource, the 3GPP LTE Release-8system is designed to divide an overall frequency region (for example, asystem bandwidth) into one or more sub-regions (for example, a physicalresource block (PRB) unit), and a predetermined scheme for exchangingICIC messages of individual frequency sub-regions between cells isdefined in the 3GPP LTE Release-8 system. A variety of parameters may becontained in an ICIC message for frequency resources. For example, aRelative Narrowband Transmission Power (RNTP) related to downlinktransmission power, uplink (UL) Interference Overhead Indication (10I)related to uplink interference, UL High Interference Indication (HII),etc. may be defined in the ICIC message for frequency resources.

RNTP is information indicating downlink transmission power used in aspecific frequency sub-region by a cell transmitting an ICIC message.For example, if an RNTP field for a specific frequency sub-region is setto a first value (for example, 0), this means that downlink transmissionpower of the corresponding cell does not exceed a predeterminedthreshold value in the corresponding frequency sub-region.Alternatively, if the RNTP field for the specific frequency sub-regionis set to a second value (for example, 1), this means that thecorresponding cell cannot promise downlink transmission power in thecorresponding frequency sub-region. In other words, if the RNTP field isset to zero ‘0’, this means that downlink transmission power of thecorresponding cell is low in the corresponding frequency sub-region.Otherwise, if the RNTP field is set to 1, this means that downlinktransmission power of the corresponding cell is not low in thecorresponding frequency sub-region.

UL ICH is information indicating the amount of uplink interferenceexperienced (or generated) in a specific frequency sub-region includinga cell configured to transmit an ICIC message. For example, if an IOIfield for a specific frequency sub-region has a high-interferenceamount, this means that the corresponding cell experiences high uplinkinterference in the corresponding frequency sub-region. In the frequencysub-region corresponding to an IOI indicating high uplink interference,the cell having received the ICIC message can schedule a UE that useslow uplink transmission power from among serving UEs of the cell.Therefore, since UEs perform uplink transmission at low transmissionpower in the frequency sub-region corresponding to an IOI indicatinghigh uplink interference, uplink interference experienced by a neighborcell (that is, a cell having transmitted the ICIC message) may bereduced.

UL HII indicates the degree of interference (or uplink interferencesensitivity) that may be encountered in the corresponding frequencysub-region because of uplink transmission within a cell configured totransmit the ICIC message. For example, if the HII field is set to afirst value (for example, 1) in a specific frequency sub-region, thereis a high possibility of scheduling a high uplink transmission power UEby a cell for transmission of the ICIC message in the correspondingfrequency sub-region. On the other hand, if the HII field is set to asecond value (for example, 0) in a specific frequency sub-region, thereis a possibility of scheduling a low uplink transmission power UE by thecell for transmission of the ICI message in the corresponding frequencysub-region. Meanwhile, if a UE is first scheduled in a frequencysub-region in which an HII is set to a second value (for example, 0) andsome UEs capable of properly operating even under high interference arescheduled in another frequency sub-region in which an HII is set to afirst value (for example, 1), one cell having received the ICIC messagecan avoid interference from another cell having transmitted the ICICmessage.

Meanwhile, as an exemplary ICIC of the time resource, the 3GPP LTE-Asystem (or 3GPP LTE Release-10) system is designed to divide an overalltime region into one or more sub-regions (for example, a subframe unit)in a frequency domain, and a predetermined scheme for exchangingspecific information indicating silencing or non-silencing of individualfrequency sub-regions between cells is defined in the 3GPP LTE-A system.The cell having transmitted the ICIC message may transmit specificinformation indicating the presence of silencing in a specific subframeto neighbor cells, and does not schedule a PDSCH and a PUSCH in thecorresponding subframe. On the other hand, the cell having received theICIC message can schedule uplink transmission and/or downlinktransmission for a UE on a subframe in which silencing is performed inanother cell having transmitted the ICIC message.

Silencing may refer to an operation of a specific cell within a specificsubframe. That is, the silencing operation indicates that a specificcell does not perform most of signal transmission on uplink or downlinkof a specific subframe. If necessary, the silencing operation may alsoindicate that a specific cell can transmit signals at no power or lowpower on uplink and downlink of a specific subframe. As an example ofthe silencing operation, a specific cell may configure a specificsubframe as a Multicast-Broadcast Single Frequency Network (MBSFN)subframe. In a downlink subframe configured as the MBSFN subframe, asignal is transmitted only in a control region and is not transmitted ina data region. As another example of the silencing operation, a cellcausing interference may configure a specific frame as a specific AlmostBlank Subframe (ABS) or an ABS-with-MBSFN. The ABS refers to a subframein which only a CRS is transmitted in a control region and a data regionof a downlink subframe and the remaining control information and dataother than the CRS are not transmitted in the control and data regionsof the downlink subframe. If necessary, signals are transmitted at nopower or low power in the subframe corresponding to the ABS.Nonetheless, downlink channels and downlink signals such as a PhysicalBroadcast Channel (PBCH), a Primary Synchronization Signal (PSS), and aSecondary Synchronization Signal (SSS) may be transmitted even in theABS. The ABS-with-MBSFN may mean a subframe in which even the CRS is nottransmitted in the data region of the above-described ABS. As describedabove, silencing may be performed in units of a specific subframe, andinformation indicating whether silencing is performed is referred to asa silent subframe pattern.

In association with ABS, ABS signaling defined in 3GPP LTE-A is largelyclassified into ABS information and an ABS status. The ABS informationindicates a subframe to be used as ABS using bitmap. The ABS informationis composed of 40 bits in case of FDD, and is composed of a maximum of70 bits in case of TDD. The number of bits used for ABS information inTDD may be changed according to UL-DL configuration. In case of FDD, 40bits indicate 40 subframes. If the value of a bit is set to 1, the bitindicates ABS. If the value of a bit is set to zero, the bit indicatesnon-ABS. When restricted measurement is configured in a UE, the numberof CRS antenna ports of the corresponding cell is notified for CRSmeasurement. A measurement subset is a subset of ABS patterninformation. The measurement subset is a bitmap composed of 40 bits incase of FDD, and is a bitmap composed of a maximum of 70 bits in case ofTDD. The above information can be understood as a restricted measurementfor configuring restricted measurement. Table 1 indicates ABSinformation defined in the legacy LTE/LTE-A system.

TABLE 1 IE type and IE/Group Name Presence Range reference Semanticsdescription CHOICE ABS M — — Information >FDD — — >>ABS Pattern M BITEach position in the bitmap represents a DL Info STRING subframe, forwhich value ″1″ indicates ‘ABS’ (SIZE(40)) and value ″0″ indicates ’nonABS’. The first position of the ABS pattern corresponds to subframe 0 ina radio frame where SFN = 0. The ABS pattern is continuously repeated inall radio frames. The maximum number of subframes is 40. >>Number Of MENUME P (number of antenna ports for cell-specific Cell-specific RATEDreference signals) defined in TS 36.211 [10] Antenna Ports (1, 2, 4, . ..) >>Measure- M BIT Indicates a subset of the ABS Pattern Info above,ment Subset STRING and is used to configure specific measurements(SIZE(40)) towards the UE. >TDD — — >>ABS Pattern M BIT Each position inthe bitmap represents a DL Info STRING subframe for which value ″1″indicates ‘ABS’ and (1 . . . 70, . . .) value ″0″ indicates ’non ABS’.The maximum number of subframes depends on UL/DL subframe configuration.The maximum number of subframes is 20 for UL/DL subframe configuration1~5; 60 for UL/DL subframe configuration 6; 70 for UL/DL subframeconfiguration 0. UL/DL subframe configuration defined in TS 36.211 [10].The first position of the ABS pattern corresponds to subframe 0 in aradio frame where SFN = 0. The ABS pattern is continuously repeated inall radio frames, and restarted each time SFN = 0. >>Number Of M ENUME P(number of antenna ports for cell-specific Cell-specific RATED referencesignals) defined in TS 36.211 [10] Antenna Ports (1, 2, 4, . ..) >>Measure- M BIT Indicates a subset of the ABS Pattern Info above,ment Subset STRING and is used to configure specific measurements (1 . .. 70, . . .) towards the UE >ABS Inactive M NULL Indicates thatinterference coordination by means of almost blank sub frames is notactive

Table 2 shows ABS status information elements (IEs) defined in thelegacy LTE/LTE-A system. The ABS status information elements are used toenable the eNB to determine whether the ABS pattern must be changed. InTable 2, ‘Usable ABS Pattern Info’ is bitmap information of a subset ofABS pattern information, and indicates whether a subframe designated asABS has been correctly used for interference reduction. ‘DL ABS status’indicates the ratio of the number of DL RBs scheduled in a subframeindicated by ‘Usable ABS Pattern Info’ to the number of RBs allocatedfor a UE to be protected through ABS. ‘DL ABS status’ may also indicatewhether ABS has been efficiently used in a victim cell according to itsown purpose.

TABLE 2 IE type and IE/Group Name Presence Range reference Semanticsdescription DL ABS status M INTEGE Percentage of used ABS resources. TheR (0 . . . 100) numerator of the percentage calculation consists ofresource blocks within the ABS indicated in the Usable ABS Pattern InfoIE allocated by the eNB₂ for UEs needing protection by ABS frominter-cell interference for DL scheduling, or allocated by the eNB₂ forother reasons (e.g. some control channels). The denominator of thepercentage calculation is the total quantity of resource blocks withinthe ABS indicated in the Usable ABS Pattern Info IE. CHOICE Usable M — —ABS Information >FDD — — >>Usable ABS M BIT Each position in the bitmaprepresents a Pattern Info STRING subframe, for which value ″1″ indicates‘ABS (SIZE(40)) that has been designated as protected from inter- cellinterference by the eNB₁, and available to serve this purpose for DLscheduling in the eNB₂’ and value ″0″ is used for all other subframes.The pattern represented by the bitmap is a subset of, or the same as,the corresponding ABS Pattern Info IE conveyed in the LOAD INFORMATIONmessage from the eNB₁. >TDD — — >>Usable ABS M BIT Each position in thebitmap represents a Pattern Info STRING subframe, for which value ″1″indicates ‘ABS (1 . . . 70) that has been designated as protected frominter- cell interference by the eNB₁, and available to serve thispurpose for DL scheduling in the eNB₂’ and value ″0″ is used for allother subframes. The pattern represented by the bitmap is a subset of,or the same as, the corresponding ABS Pattern Info IE conveyed in theLOAD INFORMATION message from the eNB₁.

A measurement subset composed of a subset of an ABS pattern is asubframe statically used as ABS, and the remaining subframes containedin the ABS pattern may determine whether a transmission point will beused as the ABS according to traffic load.

Measurement/Measurement Report

A measurement report is used for many techniques designed to ensure themobility of UEs (handover, random access, cell search, etc.) or for oneof the techniques. Since the measurement report needs a certain degreeof coherent demodulation, a UE may perform measurement after acquiringsynchronization and physical layer parameters, except for measurement ofa received signal strength. The measurement report conceptually coversRadio Resource Management (RRM) measurement of measuring the signalstrengths or signal strengths to total reception power of a serving celland neighbor cells, including Reference Signal Received Power (RSRP),Received Signal Strength Indicator (RSSI), and Reference Signal ReceivedQuality (RSRQ), and Radio Link Monitoring (RLM) measurement of measuringlink quality with respect to the serving cell to thereby determinewhether a radio link has been failed.

In association with Radio Resource Management (RRM), RSRP is defined asthe linear average over the power contributions of REs that carrydownlink CRS. RSSI is defined as the linear average of the totalreceived power of a UE. The RSSI is measured from OFDM symbols carryingRS for antenna port 0, including interference and noise power fromneighbor cells. If a specific subframe is indicated for RSRQ measurementby higher-layer signaling, the RSSI is measured over all OFDM symbols ofthe indicated subframe. RSRQ is defined as (NxRSRP/RSSI), where N is thenumber of RBs over the measurement bandwidth of RSSI.

The purpose of RLM execution is to enable a UE to monitor a downlinkquality of its own serving cell, such that the UE can determine‘in-sync’ or ‘out-of-sync’ of the corresponding cell. In this case, RLMis based on CRS. A downlink quality estimated by the UE is compared witheach of ‘in-sync threshold(Qin)’ and ‘out-of-sync threshold(Qout)’. Eachthreshold value may be denoted by a PDCCH BLER (Block Error Rate) of aserving cell. Specifically, Qout may correspond to a BLER of 10%, andQin may correspond to a BLER of 2%. Actually, Qin and Qout correspond toSINR of the received CRS. If CRS reception SINR is equal to or higherthan a predetermined level (Qin), the UE decides to attach thecorresponding cell. If CRS reception SINR is less than a predeterminedlevel (Qout), the UE declares a radio link failure (RLF).

As can be seen from the above-mentioned RSRP definition, it should bepremised that measurement reporting is performed using CRS. However,assuming that cells share the same PCID as shown in FIG. 7( b), thecells are unable to discriminate between the eels having the same PCIDon the basis of the CRS, such that it is impossible to perform RRM ofeach cell using only measurement reporting including RSRP/RSRQ based onCRS. Therefore, assuming that cells have the same PCID, it is possibleto perform additional RSRP/RSRQ measurement reporting on the basis ofCSI-RS being independently transmitted. In order to increase receptionaccuracy during CSI-RS reception of a specific cell, neighbor cells donot transmit signals to a resource element (RE) to which thecorresponding CSI-RS is transmitted, such that the neighbor cells canperform higher-accuracy measurement although a frequency of CRS-RStransmission is less than a frequency of CRS transmission. Therefore,although cells have different PCIDs, CRS-based RSRP/RSRQ measurementreporting and CSI-RS RSRP/RSRQ measurement reporting are simultaneouslyperformed, resulting in increased accuracy of a network RRM.

Another purpose of CSR-RS transmission in each cell is to perform CSIfeedback to be performed by a UE to aid scheduling of a BS (eNB) thatdetermines a rank, precoding matrix, a modulation and coding scheme(MCS) or CQI to be used for DL data transmission between thecorresponding cell and the UE. In accordance with the CoMP transmissionscheme, the UE must feed back a CSI to a downlink related to acooperative cell other than the serving cell. An excessive amount ofoverhead occurs when CSIs of all cells contained in the CoMP clusterincluding the serving cell are fed back, such that CSIs of some cells(i.e., CoMP measurement set) contained in the CoMP cluster that isvaluable in cooperative scheduling and cooperative data transmission.Deciding of the CoMP measurement set of a specific UE may be configuredby selecting cells each having an RSRP of a predetermined level orhigher. To achieve the above-mentioned operation, the UE performs RSRPmeasurement reporting of cells contained in the CoMP cluster includingthe UE. Alternatively, the BS sets configurations of CSI_RS each ofwhich will perform RSPR or RSRQ measurement to a CoMP measurement set,and informs the UE of the resultant configurations. The UE may performRSRP or RSRQ measurement of CSI-RS transmitted from cells contained inthe CoMP management set. If the measurement result satisfies a specificcondition, the UE may perform reporting.

In order to implement ICIC between CoMP clusters, a UE performs RSRPmeasurement and reporting of cells contained in a contiguous CoMPcluster, such that a network and a UE can recognize which one of cellsof the contiguous CoMP cluster gives strong interference to thecorresponding UE and can also recognize which one of cells receivesstrong UL interference from the corresponding UE.

In addition to CRS based RSRP/RSRQ measurement reporting for mobilitymanagement of UE handover, the CoMP measurement set configuration andthe CSI-RS based RSRP/RSRQ measurement reporting for ICIC aresimultaneously performed, such that accuracy and flexibility of networkRRM can be increased.

Restricted Measurement

If a cell reduces a transmission (Tx) power of a specific resourceregion, a variation width of a per-resource-region interference signalreceived by a contiguous cell is increased. If averaging of theinterference signals is achieved irrespective of a resource region, itis difficult to correctly obtain CoMP and ICIC effects. A detaileddescription thereof will hereinafter be described with reference to FIG.8.

Referring to FIG. 8, in case of a normal situation, a macro cell (macroeNB) is used as an aggressor cell of a pico cell (pico eNB). The macrocell (macro eNB) can guarantee/protect performance of the pico cell(pico eNB) using the aforementioned ABS for the pico cell or pico UE. Inmore detail, the macro cell can deboost a maximum of 9bB transmissionpower in specific subframe(s), or may not transmit signals in thespecific subframe(s), resulting in the cell range extension (CRE) effectof the pico cell. In other words, if a macro cell reduces a downlinktransmission power in the ABS, a UE located in the vicinity of a celledge of cells can recognize that performance of a picocell signal havingbeen received with a noise level or lower in a normal subframe isincreased in a manner that data can be stably received in the ABS, suchthat cell coverage of a pico cell can be actually extended.

Under this situation, restricted measurement may be used for measurementreporting. In other words, if the macro cell reduces a transmissionpower in a specific subframe through the ABS, signals and/orinterference level of the pico cell seen by the UE is greatly changedper subframe, and it is prevented that signals are simply averaged dueto introduction of the restricted measurement.

For such restricted measurement, if several CSI subframe sets (e.g., C0,C1) for channel measurement are used as a higher layer signal, the UEcan perform channel measurement and reporting dedicated for the CSIsubframe set. In addition, it is desirable that the UE may perform ABSmeasurement of the macro cell for RLM/RRM.

Cell Range Extension (CRE)

Several small-sized pico eNBs (BSs) are installed in a coverage of themacro BS, such that UEs covered by the macro BS are handed over to thepico BS, resulting in traffic dispersion of the macro BS. Handover froma serving BS to a target BS is achieved when target-BS measurementresult obtained from the UE is identical to or higher than apredetermined threshold value (Sth_conv). In this case, the networkimproves UE capability using arbitrary means, such that handover can beperformed even though signal strength (e.g., SNR) of the target BS isless than a predetermined threshold. The above-mentioned operation maybe referred to as a cell range expansion (CRE). A CRE enable region isreferred to as a CRE region/area, and the CRE region may be representedby a specific region in which a reception performance (S_(received)) ofa reference signal of the corresponding BS is higher than a newthreshold value (S_(th) _(—) _(CRE)) for CRE. That is, the CRE regionconfigures the following equation 1.

S _(th) _(—) _(conv) ≧S _(received) ≧S _(th) _(—) _(CRE)  [Equation 1]

For better understanding of the present invention, a CRE regionconfigured to satisfy Equation 1 may correspond to a shaded part.

In FIG. 9, a macro eNB enables a PUE located in the CRE region to behanded over to a pico eNB (PeNB), resulting in implementation of trafficoffloading. As a result, overall system performance is improved. The CREcan extend a cell range or a cell radius of the corresponding eNB. Inthe legacy LTE/LTE-A system, a reference signal reception intensity ofthe PeNB may be denoted by RSRP/RSRQ, a reference for enabling the UE toattach a specific cell satisfies a specific condition in which adifference between the best RSRP and a specific cell RSRP is 6 dB orless on the basis of per-cell RSRP. However, in order to increase thetraffic dispersion effect to the PeNB, the reference may be adjusted to6 dB (e.g., 9 dB) or higher. In this case, the operation (i.e., CRE) forenabling the UE to measure the PeNB, when the UE performs handover tothe PeNB and then measures the PeNB acting as a serving cell, influenceof interference caused by the macro eNB (that is located close to thePeNB and includes other BSs not shown in drawings may be furtherincreased unavoidably. Therefore, the following description discloses avariety of methods for solving various interference problems encounteredwhen a reference is higher than the CRE reference.

FIG. 10 is a conceptual diagram illustrating summaries of detailedproposals according to embodiments of the present invention.

In FIG. 10, it is premised that a UE configured to use a macro eNB as aserving cell is located in the CRE region of 6 dB or higher contained inthe pico eNB (PeNB). It is also premised that timing points (e.g., radioframe boundary or subframe boundary, etc.) of the macro eNB and the picoeNB are synchronized within the range of a predetermined degree (e.g., 3μs), or timing points of two cells are identical to each other.

In addition, the UE may have Further enhanced ICIC (FeICIC) capabilitycapable of supporting FeICIC. In this case, FeICIC means that a pico eNBperforms CRE of at least 6 dB and at the same time the macro eNb and thepico eNB perform time/frequency ICIC. There are a variety of UEcapabilities related to FeICIC capability, for example, CRS interferencecancellation (CRS IC) capability (including the number of CRS to cancel,the number of CRS capable of being cancelled in one subframe, andinformation indicating how many cells can be CRS-cancelled) capable ofperforming cancellation of CRS interference, PSS/SSS IC capability(including the number of PSSs/SSSs to cancel, the number of PSSs/SSSscapable of being cancelled in one subframe, and information indicatinghow many cells can be PSS/SSS-cancelled) capable of cancelling PSS/SSSinterference of a contiguous cell, PBCH IC capability (including thenumber of PBCHs to cancel, the number of PBCHs capable of beingcancelled in one subframe, and information indicating how many cells canbe PBCH-cancelled) capable of cancelling PBCH interference of acontiguous cell. Hereinafter, the UE capability related to FeICICcapability will be referred to as CRE-related capability. UE capabilityinformation related to CRE may be transferred from a UE to a corenetwork after RRC connection. In more detail, after the UE performs RRCconnection, the core network transmits UEcapabilityEnquiry informationto the UE through NAS (Non-Access Stratum) signaling, and the UEtransmits UE capability information in response to the receivedUECapabilityEnquiry information. If necessary, the core network maytransmit the UECapabilityEnquiry information.

Referring to FIG. 10( a), a macro eNB acting as a serving cell maytransmit a command for measuring neighbor cell(s) and a pico eNB to theUE including FeICIC capability. Therefore, if the UE measures the picoeNB, it may be impossible for the UE to measure the pico eNB withoutreceiving the help of a network because interference transmitted fromthe macro eNB occurs. Furthermore, the UE may fail to obtain picocellsynchronization (PSS/SSS reception) needed for measurement due to theoccurrence of interference from the macro eNB. Although the UE hasCRE-related capability (specifically, CRS IC capability), assuming thatthe UE does not recognize which cell of the pico eNB is a maininterference causing cell or does not recognize which one of CRSconfigurations must be cancelled, the UE may have difficulty inachieving synchronization acquisition/measurement.

In relation to the above-mentioned problem, the embodiment of thepresent invention will disclose signaling, process, etc. needed for theUE having obtained synchronization of a neighbor cell including highinterference under the above-mentioned situation.

If the UE measures a neighbor cell under high interference and receivesa handover command of a serving cell for a high-level (6 dB or higher)CRE, the UE needs to receive a PBCH of the neighbor cell when beinghanded over to the neighbor cell (i.e., state transition from FIG. 10 ato FIG. 10 b). However, the UE may have difficulty in receiving a PBCHof the neighbor cell due to a PBCH of a cell acting as an interferencesource (macro eNB of FIG. 10 b). The above-mentioned operation may alsobe applied to the case of receiving other system information withoutchange. In association with the above-mentioned description, signaling,process, etc., needed for correctly transferring system informationincluding a PBCH to the UE will hereinafter be described in detail.

Continuously, according to the embodiment of the present invention, theUE may have difficulty in measuring the serving cell even when handoverto the neighbor cell (e.g., pico eNB of FIG. 10 a) under highinterference. In other words, there is a high possibility that a UEconfigured to use a high-interference cell (e.g., pico eNB of FIG. 10 b)as a serving cell has difficulty in correctly measuring its own servingcell due to a neighbor cell (e.g., a macro eNB of FIG. 10 b) causinghigh interference. Therefore, the embodiment of the present inventionwill disclose signaling, process, etc. related to the serving cellmeasurement to solve the above-mentioned problems.

In summary, the following description will sequentially disclose: i)acquisition of synchronization of a neighbor cell under highinterference; ii) measurement of a neighbor cell under highinterference; iii) acquisition of system information of a neighbor cellunder high interference; and iv) measurement of a serving cell underhigh interference.

In the following description, the term ‘measurement’ refers tomeasurement of at least one of RRM/RLM/CSI unless specially noted, andeach of the neighbor cell receiving interference and the serving cellreceiving interference may be referred to as a weak cell or a victimcell, and a cell causing interference may be referred to as an aggressorcell.

Synchronization Acquisition of Neighbor Cell Under High Interference

In accordance with a method for synchronization acquisition of aneighbor cell (i.e., a weak cell) receiving high interference fromneighbor cells, if the UE has FeICIC capability and PSS/SSS ICcapability, the eNB (BS) may allow the UE to directly receive PSS/SSS ofthe weak cell such that the UE can acquire necessary information. Inmore detail, the eNB gives the list of weak cells (i.e., the list ofneighbor cells to be described later) to the UE, and may command the UEto perform cell acquisition of the corresponding cell using the PSS/SSSIC. In this case, information indicating who an aggressor cell for weakcells is may be omitted as necessary, and the UE can detect PSS/SSS ofthe weak cell without using information of the aggressor cell.

In accordance with another method, a serving cell may inform a UE havingFeICIC capability of specific information indicating which aggressorcell is synchronized with a specific weak cell, and the serving cell maycommand the UE having received such information to perform CRSmeasurement without detecting PSS/SS of the corresponding cell. That is,in order to measure a neighbor cell of a UE by the eNB, the list of weakcells to be measured by the UE should be transferred, and RSRP/RSRQ ofthese cells can be measured only in a specific subframe. In addition,information indicating which macro eNB has been synchronized in thesecells is provided. In this case, provision of the information regardingsynchronization with a certain macro eNB may enable the UE to measureRSRP/RSRQ without detecting PSS/SSS of a weak cell, because a signalintensity is high in a manner that the UE can perform PSS/SSS detectionand specific weak cells inform the UE of the same radio frameboundary/subframe boundary as those of the corresponding macro eNB.

In this case, it is impossible to recognize TDD/FDD information, CP typeinformation, etc. capable of being recognized in PSS/SSS detection. Forthis purpose, information regarding the macro eNB having an aggressorrelationship per weak cell is transferred to the UE. In this case, themacro eNB may have the same duplex (TDD/FDD) scheme as those of thecorresponding weak cell, and may have the same CP type as in thecorresponding weak cell. In association with the CP type, a network(macro eNBs configured to use the ABS and the pico eNBs configured toreceive the help) in which the ABS is configured may have the same CPtype. The UE may operate on the assumption that the macro eNB and thepico eNB have the same CP type. Alternatively, the multiplexing schemesof individual weak cells, the CP type, and other system information maybe further notified as necessary.

Measurement of Neighbor Cell Under High Interference

A UE having received a command for measuring a neighbor cell (weak cell)serving has to measure a reference signal of the corresponding cellafter synchronization acquisition of weak cells. From the viewpoint ofCRS, the UE measures intensity of CRS reception signals of neighborcells and must report the measurement result to the eNB. The CRSreception intensity may be represented by RSRP/RSRQ. In order tocorrectly measure the CRS of each weak cell, it is necessary for the UEto properly process CRS interference of eNBs.

Provided that FeICIC capability is set and CRS IC capability is set, theeNB can transmit the list of weak cells to be measured to thecorresponding UE. When the eNB transmits a measurement execution commandto the UE, the eNB can transmit reference signal information (e.g., CRSinformation) of a neighbor cell (aggressor cell, for example, a macroeNB adjacent to a pico cell) causing interference to cell(s) containedin the weak-cell list. In this case, CRS information of contiguous macroeNBs may include a cell ID, a CRS port number, and information oftime/frequency domains to which CRS is transmitted. Detailed informationrelated to RS information will be disclosed later. When the UE havingreceived the above information measures a specific cell contained in theweak-cell list, RS cancellation is performed using CRS information ofthe specific cell, such that the UE may report the measurement resultfrom which influence of interference is removed. If the eNB performsPDSCH mapping through CRS rate-matching and transmits CRS information toinform the UE of the mapped result, the eNB may transmit CRS informationof a neighbor cell irrespective of the CRS IC capability of a UE,differently from the above-mentioned explanation.

FIGS. 11 and 12 are flowcharts illustrating a method for enabling theeNB to transmit a measurement command. Referring to FIG. 11, the eNBconfirms UE capability information in step S1111. In this case, the UEcapability information relates to CRE related capability, such as FeICICcapability, CRS IC capability, etc. If the eNB does not have the UEcapability information, the eNB may perform the UE capabilityinformation exchange procedure. If the UE has FeICIC capability in stepS1113, it is determined whether the UE has CRS IC capability in stepS1115. Assuming that the UE has CRS IC capability, when the eNB commandsthe UE to measure a weak cell, the eNB may include i) the list of weakcells to be measured, ii) a subframe pattern which will measure a weakcell, iii) relationship between an aggressor cell and a victim cell foreach weak cell, iv) system information of a weak cell (e.g., duplextype, CP length, etc.), and v) CRS information of an aggressor cell forweak-cell measurement.

FIG. 12 is a flowchart illustrating a method for signaling CRSinformation of a neighbor cell on the basis of UE capabilityinformation. Referring to FIG. 12, steps other than step S1217 arereplaced with those of FIG. 11. If the UE has FeICIC capability and CRSIC capability, the UE certainly provides CRS information of aggressorcells of weak cells, and requires measurement of each weak cell. In thiscase, since there is a high possibility that UE battery power can beexcessively consumed, the eNB may command the corresponding UE tomeasure a weak cell as necessary. For example, if the corresponding UEmay be located in the CRE region of a neighbor pico eNB, the eNB maycommand the UE to measure the weak cell. Under the condition that theeNB has difficulty in correctly recognizing whether the UE is located inthe CRE region, if specific events i), ii) and iii) occur (where i) aCQI level reported by a UE is continuously equal to or less than apredetermined CQI level, ii) successive errors occur in DL data for thecorresponding UE or UL data, iii) a UE average throughput is equal to orless than a predetermined throughput), the eNB can estimate that thecorresponding UE has been located in the CRE region.

Therefore, the eNB may command the corresponding UE to measure a weakcell to perform handover to a neighbor pico eNB, and may transmitnecessary signaling information and CRS information of a neighbor cell.FIG. 13 is a flowchart of a process from among processes in which theeNB can recognize that the corresponding UE is located in the CRE regionunder the condition that a CQI level reported by the UE is continuouslyequal to or less than a predetermined CQI level. In S1311 to S1315, if aCQI level reported by the UE is equal to or less than a threshold valueand reaches a predetermined value (N) or higher, the eNB determines theUE capability in step S1317. If the UE has FeICIC capability and CRS ICcapability, the UE may transmit a weak cell measurement command in stepS1317. Detailed description of information that is capable of beingcontained in the weak cell measurement command or being simultaneouslytransmitted is based on step S1117 of FIG. 11.

The above-mentioned measurement command will hereinafter be described indetail.

If the FeICIC capability is set in the UE-EUTRA-Capability informationelement (if the corresponding UE can perform FeICIC), the eNB maycommand the corresponding UE to measure a reference signal of eachneighbor pico eNB having low intensity of a reception signal of thecorresponding UE. In the network in which ABS is configured, the eNB maycommand the UE to perform restricted measurement. In this case, in orderto perform the restricted measurement in view of a subframe, differentsubframe sets for serving cell CQI reporting are configured, and thelist of cell IDs and the measurement subframe pattern for the cell IDlist may be designated for restricted RRM/RLM measurement of a neighborcell.

For example, the eNB may indicate restricted RRM/RLM measurement throughMeasObjectEUTRA information element (IE) including subfields shown inthe following Table 3.

TABLE 3 MeasSubframePatternConfigNeigh ::=   CHOICE {    release NULL,   setup SEQUENCE {       measSubframePatternNeighMeasSubframePattern-r10,       measSubframeCellListMeasSubframeCellList-r10    OPTIONAL  -- Cond measSubframe      aggressorCellCRSinformation {          Cell ID, Number of CRSports, CRS transmission BW, center frequency, subframes containing CRSin the data region }    } }

In Table 3, ‘measSubframePatternNeig’ may denote a subframe pattern thatmust perform neighbor cell RSRP/RSRQ measurement, and‘measSubframeCellList-r10’ may denote the list of cells to which‘measSubframePatternNeig’ is applied. That is, the eNB may allocate asubframe pattern to be used for RSRP/RSRQ measurement of a specific celllist to the UE through a sub-element shown in Table 3. Preferably, thesubframe pattern must be designated as a subframe pattern, that is setto ABS by the macro eNBs, to increase RSRP/RSRQ performance of the picoeNB. The cell contained in ‘measSubframeCellList’ information may be aweak cell under high interference, and ‘aggressorCellCRSinformation’information may be RS information causing high interference to a cellcaontained in the measSubframeCellList information.

Configuration of restricted measurement for neighbor cell RRMmeasurement may indicate that ‘MeassubframePatternNeigh’ information fora neighbor cell RRM has been configured. In this case, assuming thatadditional aggressor cell CRS information is not present, the UEperforms RRM only in a subframe designated in the‘MeassubframePatternNeigh’ information of cells contained in the‘meassubframeCellList’ information. However, upon receiving theadditional aggressor cell list and CRS information(aggressorCellCRSinformation) of the corresponding cells, the UEperforms RRM only in a subframe designated in ‘MeassubframePatternNeigh’information in association with the cells contained in themeassubframeCellList information, and then reports a value (e.g.,measured RSRQ/RSRP) obtained after CRS interference of aggressor cellsis reduced to a serving cell of the UE.

In more detail, the RRM operations are as follows. If the restrictedmeasurement for RRM is configured and CRS information of aggressor cellsis received, and if the CRS transmission position of a target cell whichwill attempt to perform RRM in a non-MBSFN subframe overlaps andcollides with a CRS transmission position of a specific cell containedin ‘aggressorCellCRSinformation’ (i.e., Colliding CRS case), CRS of thespecific cell is cancelled to measure RSRP of the target cell to bemeasured, and the RSRP value is then calculated and reported. That is,the CRS quality obtained after interference of the colliding CRS isreduced is defined as RSRP and the defined RSRP is then reported.

However, for RSSI measurement, CRS IC of the aggressor cells is notalways requested for RSSI measurement. In other words, CRS IC of theaggressor cells may be omitted as necessary. If the restrictedmeasurement is configured, RSSI has been defined to perform electricfield strength measurement and averaging within all symbols of aspecific subframe in which RRM measurement is needed, such that CRS ICneed not be performed. Therefore, in the case of calculating RSRQ (whereRSRQ=N*RSRP/RSSI), CRS cancellation of a specific cell is performed in anumerator of RSRQ such that CRS influence of the aggressor cell isexcluded. In the case of calculating RSSI, influence of CRS of aggressorcells remains unchanged.

The above-mentioned RSRP and RSRQ measurement can be applied to the casein which RRM of the corresponding target cell is performed under thecondition that restricted measurement is established for RRM of aspecific target cell and CRS information of aggressor cells of thecorresponding target cell is signaled.

In addition, ‘aggressorCellCRSinformation’ may include serving cellinformation. That is, a serving cell of a specific neighbor cell may beused as an aggressor. In this case, when the UE performs weak cell RRM,the UE must report the value obtained after CRS IC of the serving cellis executed to the serving cell. However, if the UE has already receivedserving cell information as another container, the UE need not performadditional signaling of the serving cell CRS information, such that itcan report a value obtained after CRS IC of the serving cell requiringrestricted RRM measurement is performed, through a serving cell ID orsimple signaling information.

For example, if the remaining information other than a cell ID is missed(lost) in ‘aggressorCellCRSinformation’ and the corresponding cell ID isidentical to the serving cell ID, the UE may reduce interference of theserving cell CRS for neighbor cell RRM on the basis of the serving cellinformation already known to the UE. In addition, if information of acell ID corresponding to the serving cell ID in the‘aggressorCellCRSinformation’ information is different from serving cellinformation received through another higher layer signaling (e.g.,dedicated RRC configuration or common RRC configuration), theinformation received through another higher layer signaling may havepriority.

In another example, if restricted measurement for neighbor cell RRM isconfigured, the operation for reporting RSRP/RSRQ values obtained afterexecution of IC of the serving cell CRS may be established as a default.This reporting operation is requisite for the case in which a CRS of aspecific neighbor cell requiring RRM measurement overlaps and collideswith a CRS of the serving cell in time/frequency domains. Specifically,for RSRP, the value obtained after execution of IC of the serving cellCRS must be reported/calculated.

If restricted measurement for the neighbor cell RRM is not configured,‘aggressorCellCRSinformation’ information (i.e.,‘MeassubframePatternNeigh’ information) may not be signaled to a UE. Inother words, ‘aggressorCellCRSinformation’ information can be signaledonly when ‘MeassubframePatternNeigh’ information is configured.

In order to deliver ‘aggressorCellCRSinformation’ information when theserving cell commands the UE to perform neighbor cell RRM, it isnecessary for a plurality of eNBs to first exchange information of theaggressor cell or information the victim cell with one another. For thisoperation, the eNB acting as each victim cell can transmit the list ofits own aggressor cells to neighbor eNBs through a communicationinterface between eNBs. When the eNBs having received the aggressor celllist commands the UE to measure each weak cell, the eNBs performsignaling of the aggressor cell list of the corresponding weak cells tothe UE, such that the UE can report an RSRQ value obtained afterreduction of CRS interference of cells acting as aggressors, i.e., anRSRQ value obtained after cancellation of CRS of aggressor cells. Inanother scheme, individual eNBs may exchange the list of victim cells,each of which receives serious interference from the eNBs, through an X2interface, such that the eNBs may share information indicating whetherwhich cell is operated as an aggressor of each victim cell.

Acquisition of System Information of Neighbor Cell Under HighInterference

The eNB may enable the UE to be handed over to a specific pico eNB. Forhandover to the pico eNB, the macro eNB may further transmit a varietyof information to the UE. For example, system information of thecorresponding pico eNB, master information block (MIB) information ofthe LTE system, SFN of the corresponding cell, and an SFN differencebetween the macro eNB and the current serving macro eNB can beadditionally transmitted from the macro eNB to the UE. (Since SFN may bedifferent from SFN obtained through a PBCH of a pico cell, the UE maytransmit not only SFN subframe offset/radio frame offset but also OFDMsymbol offset/sample offset information between two cells so as toindicate a correct timing offset between two cells) It may be impossibleto decode a PBCH of the pico eNB due to interference caused by a PBCH ofthe macro eNB, and the macro eNB previously transmits system informationof a target cell, MIB and SFN/SFN offset transmitted through a PBCH tothe UE either in case of handover or prior to execution of suchhandover, such that the corresponding UE can be handed over to the picoeNB without decoding a PBCH of the pico eNB.

Meanwhile, assuming that a UE to be handed over to a specific pico eNBby the macro eNB has PBCH IC capability, the macro eNB need notpre-deliver at least information transmitted through a PBCH from amongsystem information of the pico eNB to the UE. If the UE has PBCH ICcapability, the UE can be handed over to the pico eNB such that it candirectly decode a PBCH of the pico eNB.

That is, a method for acquiring system information of a target cell by aUE performing handover may be determined according to UE CRE relatedcapability (specifically, PBCH IC capability).

However, in this case, the ratio of PBCH power to CRS power of thecorresponding pico eNB (i.e., the ratio of CRS transmission power toPBCH transmission power of the corresponding pico BS) and the ratio ofPBCH power of a neighbor cell to PBCH power of a serving cell must berecognized by the UE. The above-mentioned PBCH to CRS power ratio may becontained in a handover command and then transmitted. However, the scopeor spirit of the present invention is not limited thereto, and the PBCHto CRS power ratio may be signaled separately.

In accordance with another embodiment, instead of using signaling of thePSS/SSS/PBCH-to-CRS power ratio of its own cell and a neighbor cell, theUE may assume the PSS/SSS-to-PBCH transmission power ratios of its owncell and a neighbor cell in a manner that the UE can recognize thedegree of interference of the neighbor cell. For example, the UE mustassume that corresponding channels are transmitted with either the sametransmission power or a difference of a predetermined level(delta_power) compared with CRS, and such information may be promised asa higher layer signal. Signaling of more detailed power ratio will bedescribed later.

Meanwhile, after the UE is successfully handed over to the pico eNB, ifMIB contained in a PBCH of the pico eNB is changed and there is no PBCHIC capability, it is very difficult for the UE to recognize the changedMIB. Assuming that the UE does not recognize the changed MIB, thecorresponding UE may experience continuous errors intransmission/reception of data and control information. For example,although PHICH configuration contained in MIB is changed, the UE failsin successful reception of PBCH. Thus, if the UE does not recognize thechanged MIB, the corresponding UE may experience continuous errors. Inthis case, the UE must carry out a specific procedure for querying thepico eNB whether MIB was changed. Therefore, the UE transmits an errorreport to the pico eNB, such that this information may be used as anindicator for indicating the problems generated when the correspondingUE receives services. The error report may be used as a signalindicating that a problem unknown to the UE is associated with the UEitself. As a result, the error report may be used as a signal forquerying the eNB whether system information is changed. Upon receivingthe error report, the eNB transmits the changed system information tothe UE only when system information is changed. If system information isnot changed, the eNB may indicate no change of system information or mayalso indicate no change of system information using an acknowledgement(ACK) message. A valid time is present per system information. In thiscase, the UE may request specific information indicting whether systeminformation has been changed for a specific time duration from the eNB.In this case, information regarding the specific time duration must betransmitted to the eNB.

In another example related to change of system information, a UE mayseparately request each system information from the eNB. In case of a UElocated in the CRE region (i.e., a UE which uses a pico eNB as a servingcell and is located at a specific position having high CRE bias of thepico eNB), the UE may have difficulty in correctly receiving systeminformation of the pico eNB. However, if the UE is located within apredetermined range from a pico cell, namely, if the UE is located closeto the pico eNB, the UE may receive some system information. The UE mayrequest specific system information from the eNB. If the UE transmitsthe MIB request to the eNB, the eNB must transmit the MIB to thecorresponding UE over a PDSCH. In this case, MIB is CRC-masked with UE'sC-RNTI but not SI-RNTI, such that the CRC-masked result is sent to thecorresponding UE through dedicated transmission.

In addition, a transmission start time of the paging and SIB 1 (SystemInformation Block 1) information to be transmitted by the macro eNB maybe identical to that of the pico eNB in so far as coordination ofseparate subframe number arrangement between the macro eNB and the picoeNB is not performed through a channel assigned a decided subframenumber. In this case, paging and SIB 1 transmitted from the pico eNB mayreceive serious interference in the same manner as in a PBCH. Therefore,the UE may transmit the paging and SIB 1 request to the eNB. Theabove-mentioned system information request may not be limited to MIB,paging, and SIB 1.

In another embodiment, the UE may request only information changedwithin a specific time interval (i.e., only newly updated systeminformation) from the eNB. Likewise, the eNB having received the aboveinformation transmits only updated system information, performs CRCmasking with a C-RNIT of a UE requesting system information, andtransmits the CRC masking result to the corresponding UE over a PDSCH.If an aggregate of UEs requesting such information is present, the eNBmay perform multicasting of a group of specific UEs.

From among the above-mentioned information, if the macro eNB (sourceeNB) before handover transmits system information of the pico eNB(target eNB) to the UE, the UE may assume that two cells maintainsynchronization within a predetermined range (e.g., within the CPlength, or within a specific time (within 3 μs)). Alternatively, the UEmay explicitly indicate that two cells maintain synchronization within apredetermined range. In this case, the UE may perform synchronizationand time/frequency tracking through CRS of a pico cell or another cellmaintaining synchronization with the corresponding pico cell. However,if a timing jitter or low time accuracy are decided, the timing jittermust be tracked on the basis of a CRS of the pico cell.

If timing points of two cells always collide with each other, i.e., ifPSS/SSS and/or CRS of the two cells are always transmitted in the sametime-frequency resource region, the BS informs the UE that PSS/SSSand/or CRS always maintain synchronization. Specifically, theabove-mentioned case is more important in the case in which the UE hasto perform tracking using a CRS of the macro eNB.

Although assumption of synchronization and synchronization signalingrelated to only two cells have been disclosed for convenience ofdescription and better understanding of the present invention, it shouldbe noted that the above-mentioned assumption and signaling can also beequally or similarly applied to a plurality of cells as necessary.

Execution of Measurement of Serving Cell Under High Interference

If FeICIC capability is set in UE-EUTRA-Capability information element(if the corresponding UE is configured to perform FeICIC), and if CRS ICcapability is set, the serving cell can inform the UE of not only thelist of aggressor causing serious interference to the serving cellcells, but also CRS information of each cell. However, information as towhether the UE will perform CRS IC on certain measurement (i.e.,information as to whether CRE-related capability of a UE was used) maybe determined in different ways according to a measurement target, thepresence or absence of UE capability, restricted measurementconfiguration, etc. Hereinafter, the case (i) for RRM measurement, thecase (ii) for RLM measurement, and the case (iii) for restrictedmeasurement will be sequentially described.

In case of RRM measurement, in the legacy LTE/LTEOA system, RRM/RLM ofthe serving cell is performed only in a given subframe(MeassubframePattern) when ‘MeassubframePatternPCell(-r10)’ is receivedin ‘RadioResourceconfigDedicated IE’. If restricted measurement for theserving cell RRM/RLM is configured as described above, and if the UEFeICIC capability is set, the serving cell informs the UE of CRSinformation of each cell operated as an aggressor, resulting inincreased RRM/RLM performance. The UE having received the above CRSinformation performs CRS IC of these cells, and then performs RRM/RLM.In this case, detailed information of the above CRS information mayrefer to the signaling part of RS information to be described later.

Meanwhile, although the UE receives CRS information of aggressor cellsfrom the eNB, if restricted measurement for RRM/RLM is not configured,i.e., if the UE does not receive a separate measurement pattern forRRM/RLM, the UE does not perform any other IC operations related to CRSinterference of a neighbor cell when performing RRM/RLM of the servingcell.

A more detailed description of the RRM operation is as follows. Assumingthat restricted measurement for RRM is configured and CRS information ofaggressor cells is received, if the CRS transmission position of atarget cell that attempts to perform RRM in a non-MBSFN subframeoverlaps and collies with (i.e., Colliding CRS case) the CRStransmission position of a specific cell contained in‘aggressorCellCRSinformation’, CRS of the specific cell is cancelled tomeasure RSRP of the target cell to be measured, and the RSRP value isthen calculated and reported. That is, the CRS quality obtained afterinterference of the colliding CRS is reduced is defined as RSRP and thedefined RSRP is then reported.

However, for RSSI measurement, CRS IC of the aggressor cells is notalways requested for RSSI measurement. In other words, CRS IC of theaggressor cells may be omitted as necessary. If the restrictedmeasurement is configured, RSSI has been defined to perform electricfield strength measurement and averaging within all symbols of aspecific subframe in which RRM measurement is needed, such that CRS ICneed not be performed. Therefore, in the case of calculating RSRQ (whereRSRQ=N*RSRP/RSSI), CRS cancellation of a specific cell is performed in anumerator of RSRQ such that CRS influence of the aggressor cell isexcluded. In the case of calculating RSSI, influence of CRS of aggressorcells remains unchanged.

If a specific UE has interference cancellation capability, RLM of thecorresponding UE must be carried out on the basis of SINR obtained aftercompletion of cancellation of CRS interference. Although interference ishigher than a predetermined interference level, data and controlinformation received by the corresponding UE can be stably demodulatedand decoded because the corresponding UE performs cancellation ofinterference.

Specifically, if the CRS of several cells collide with each otherbecause CRS positions are transmitted from the same REs as those of theneighbor cells, the UE first performs cancellation of a neighbor cellCRS used as dominant interference. Thereafter, the UE determines howmuch SINR of a CRS received from its own serving cell is good, such thatit may determine whether to continuously maintain synchronization withthe corresponding cell, or may decide to declare Radio Link Failure(RLF). However, CRS SINR obtained after all CRS of neighbor cells arecancelled are unable to reflect realistic SINR in an RE other than CRS.Interference may be removed from the RE mapped to CRS, such that SINR ofthe RE can be improved. However, in fact, interference from neighborcells may still remain in an RE mapped to a PDCCH, such that it isimpossible for the CRS SINR to reflect realistic SINR in the RE.thereafter, after completion of CRS interference cancellation, thedegree of neighbor-cell interference applied to the RE mapped to a PDCCHneeds to be recognized by the UE, such that the UE can recognize asubstantial interference level and can perform more accurate RLM.

In conclusion, for reliable RLM, the PDCCH-to-CRS power ratio of aneighbor cell must be signaled to the UE. The UE cancels CRS using suchinformation, and then performs RLM by referring to the resultant value.That is, when measuring/calculating CRS SINR for RLM, the remaininginformation amount obtained after completion CRS interferencecancellation of a neighbor cell is calculated in consideration of thePDCCH-to-CRS power ratio of a neighbor or contiguous cell.

The PDCCH-to-CRS power ratio of the neighbor cell can be transmitted asa higher layer signal. Specifically, if the eNB transmission power isdivided into a plurality of subframe sets having different transmissionpowers in the same manner as in ABS, the PDCCH-to-CRS power ratio of theeNB per set must be signaled.

If restricted measurement for CSI measurement is configured, i.e., if asubframe pattern for CSI reporting is configured, the UE measures a CSIper subframe pattern and reports the measured CSI. In the followingdescription, it is assumed that ‘csi-MeassubframeSet1’ related torestricted measurement is a subframe set obtained when neighbor macroeNBs reduce interference through ABS operations or the like, and‘csi-MeassubframeSet2’ is a subframe set obtained when interference fromneighbor macro eNBS is not reduced. Restricted CSI measurement isconfigured in the UE, and the serving cell can inform the UE of CRSinformation of neighbor aggressor cells. The UE having received theabove information performs CRS IC of neighbor aggressor eNBs in thesubframe set (csi-MeassubframeSet1) corresponding to a subframe in whichthe neighbor aggressor eNBs can reduce interference using ABS operationsor the like, such that the UE may report more improved CSI. However,although the UE has CRS IC capability in the other subframe set(csi-MeassubframeSet2) used as a normal subframe in which the operationfor reducing transmission power of neighbor aggressor cells is notperformed, CRS IC is not performed in the corresponding subframe set. Inmore detail, although CRS interference is cancelled, interferencegenerated in data REs other than CRS is not reduced, so that the UE doesnot perform the CRS IC in the corresponding subframe set.

Although CRS information of neighbor aggressor cells is received, if‘csi-subframePattern’ information is not configured in the UE, i.e., ifthe UE does not receive a plurality of CSI measurement subframe patterns(e.g., csi-MeassubframeSet1 and csi-MeassubframeSet2), the UE may notperform CRS IC for CSI measurement.

Alternatively, although ‘csi-subframePattern’ information is configuredin the UE, i.e., although the UE receives a plurality of CSI measurementsubframe patterns (e.g., csi-MeassubframeSet1 and csi-MeassubframeSet2)and does not receive CRS information of each neighbor aggressor cell,the UE may not perform the CRS IC operation for CSI measurement even ata certain CSI measurement subframe set. Provided that the UE performsthe CRS IC operation at a specific subframe set for CSI measurement, theUE informs the eNB of CRS IC execution when calculating the CSI value,and can inform the eNB of information indicating which one of subframesets has been used to carry out the above operation.

In order to configure, by the UE, ‘csi-subframePatternconfig’information and to receive the CSI report after execution of CRS IC at aspecific subframe set by providing CRS information of the aggressorcells, the eNB may inform the UE of information indicating whichsubframe sets is a subframe set that must be CRS-IC-processed by the UE.Alternatively, the above-mentioned information may be promised in theorder of subframes signaled between the eNB and the UE. For example, ifCRS information is given in ‘csi-MeassubframeSet1’, thecsi-MeassubframeSet1 may be promised to be a subframe set, CRS IC ofwhich is needed. In case of csi-MeassubframeSet2, thecsi-MeassubframeSet2 may be promised to be a subframe set, CRS IC ofwhich is no longer required.

FIG. 14 is a flowchart illustrating measurement of the serving cell.Referring to FIG. 14( a), if the eNB acting as the serving cellrecognizes UE capability information (CRE related capabilityinformation), the eNB confirms the UE capability information. If thereis no UE capability information, a procedure for exchanging the UEcapability information may be performed. If the UE has FeICIC capabilityin step S1413, it is determined whether the UE has CRS IC capability instep S1415. If the UE has CRS IC capability, RS information of aggressorcells may be transferred to the UE. FIG. 14( b) is basically similar toFIG. 14( a). However, if restricted RRM/RLM configuration or restrictedCSI measurement configuration is further considered and satisfied instep S1427, RS information of the aggressor cells may be signaled.

Signaling of Power Ratio

If the UE has a interference cancellation receiver, the eNB may informthe UE of the PSS-to-CRS power ratio of its own cell and neighbor cell,the SSS-to-CRS power ratio, and the PBCH-to-CRS power ratio, the UE mayremove interference such as PSS/SSS/PBCH of the cell causing suchinterference, such that handover to a weak cell can be smoothly carriedout. In this case, estimation of channels (such as PSS/SSS and PBCH) isachieved on the basis of CRS, the PSS/SSS to CRS power ratio and thePBCH to CRS power ratio must be signaled to the UE. Specifically, if adominant interference source of the target cell for handover is a cellcurrently attached to the UE, the eNB can transmit the PSS/SSS/PBCH toCRS power ratio of its own cell.

In accordance with another embodiment, instead of using signaling of thePSS/SSS/PBCH-to-CRS power ratio of its own cell and a neighbor cell, theUE may assume the PSS/SSS-to-PBCH transmission power ratios of its owncell and a neighbor cell in a manner that the UE can recognize thedegree of interference of the neighbor cell. For example, the UE mustassume that corresponding channels are transmitted with either the sametransmission power or a difference of a predetermined level(delta_power) compared with CRS, and such information may be promised asa higher layer signal. Preferably, in order to perform not onlyperformance improvement of a channel reception signal from the UEserving cell but also interference cancellation of the reception signal,the BS may perform signaling of per-channel CRS power ratios of its owncell and a neighbor cell as necessary.

As an example for reliably receiving/demodulating PSS/SSS/PBCH in aserious interference environment, the UE may detect a cell only in asubframe established as ABS by macro eNBs. That is, when ABS patternsare exchanged between the macro eNB and the pico eNB, subframe offsetinformation between individual eNBs is also exchanged, such that asubframe for PSS/SSS/PBCH transmission of the macro eNB may not collidewith a subframe for PSS/SSS/PBCH of the pico eNB. In addition, the macroeNBs exchange the ABS patterns with one another in consideration of theabove information, and each macro eNB may certainly set the subframerequisite for the pico eNB configured to transmit PSS/SSS/PBCH to theABS as necessary. The UE may attempt to detect PSS/SSS/PBCH only in asubframe established as ABS by the macro eNBs.

Signaling of RS (Reference Signal) Information

For CRS interference handling of a UE, RS information signaled by theeNB may include a cell ID of a neighbor cell, the number of CRS ports,and time/frequency information requisite for CRS transmission.Time-related information may be denoted by a subframe in which CRS istransmitted, and its associated signaling may be MBSFN subframe setting.Frequency-related information requires a center frequency and bandwidthof each neighbor cell, the number of PRBs for CRS transmission, and thePRB position information. A message ‘NeighborCellCRSInformation’requisite for transmission of such information may be defined as thefollowing table 4.

TABLE 4 NeighborCellCRSInformation ::=  CHOICE {  Cell ID     { numberof CRS ports,      frequency information of CRS transmission,      timeinformation of CRS transmission }  }

Multiple cell IDs may be transmitted in ‘NeighborCellCRSInformation’,the number of CRS ports per cell ID, transmission of frequency CRSinformation, and time information of CRS transmission may betransmitted. Whereas the number of CRS ports per specific cell ID mustbe transmitted, frequency CRS information transmission and CRStransmission time information may be transmitted as necessary. In thiscase, the frequency CRS information transmission may be frequencyinformation related to CRS transmission, and may be denoted by a centerfrequency and bandwidth of a specific cell, or the number of PRBs forCRS transmission and the PRB position information. The term ‘timeinformation’ for CRS transmission may be time information related to CRStransmission, and may be denoted by a subframe in which CRS istransmitted. For example, the time information may be set to MBSFNsubframe configuration.

FIG. 15 is a block diagram illustrating a transmission point apparatusand a UE apparatus according to embodiments of the present invention.

Referring to FIG. 15, the transmission point apparatus 1510 according tothe present invention may include a reception (Rx) module 1511, atransmission (Tx) module 1512, a processor 1513, a memory 1514, and aplurality of antennas 1515. The plurality of antennas 1515 indicates atransmission point apparatus for supporting MIMO transmission andreception. The reception (Rx) module 1511 may receive a variety ofsignals, data and information on an uplink starting from the UE. The Txmodule 1512 may transmit a variety of signals, data and information on adownlink for the UE. The processor 1513 may provide overall control tothe transmission point apparatus 1510.

The processor 1513 of the transmission point apparatus 1510 according toone embodiment of the present invention can process the above-mentionedembodiments.

The processor 1513 of the transmission point apparatus 1510 processesinformation received at the transmission point apparatus 1510 andtransmission information to be transmitted externally. The memory 1514may store the processed information for a predetermined time. The memory1514 may be replaced with a component such as a buffer (not shown).

Referring to FIG. 15, the UE apparatus 1520 may include an Rx module1521, a Tx module 1522, a processor 1523, a memory 1524, and a pluralityof antennas 1525. The plurality of antennas 1525 indicates a UEapparatus for supporting MIMO transmission and reception. The Rx module1521 may receive downlink signals, data and information from the BS(eNB). The Tx module 1522 may transmit uplink signals, data andinformation to the BS (eNB). The processor 1523 may provide overallcontrol to the UE apparatus 1520.

The processor 1523 of the UE apparatus 1520 according to one embodimentof the present invention can process the above-mentioned embodiments.

The processor 1523 of the UE apparatus 1520 processes informationreceived at the UE apparatus 1520 and transmission information to betransmitted externally. The memory 1524 may store the processedinformation for a predetermined time. The memory 1524 may be replacedwith a component such as a buffer (not shown).

The specific configurations of the transmission point apparatus and theUE apparatus may be implemented such that the various embodiments of thepresent invention are performed independently or two or more embodimentsof the present invention are performed simultaneously. Redundant matterswill not be described herein for clarity.

The description of the transmission point apparatus 1510 shown in FIG.15 may be applied to the eNB (BS), or may also be applied to a relaynode (RN) acting as a DL transmission entity or UL reception entitywithout departing from the scope or spirit of the present invention. Inaddition, the description of the UE apparatus 1520 may be applied to theUE, or may also be applied to a relay node (RN) acting as a ULtransmission entity or DL reception entity without departing from thescope or spirit of the present invention.

The above-described embodiments of the present invention can beimplemented by a variety of means, for example, hardware, firmware,software, or a combination thereof.

In the case of implementing the present invention by hardware, thepresent invention can be implemented with application specificintegrated circuits (ASICs), Digital signal processors (DSPs), digitalsignal processing devices (DSPDs), programmable logic devices (PLDs),field programmable gate arrays (FPGAs), a processor, a controller, amicrocontroller, a microprocessor, etc.

If operations or functions of the present invention are implemented byfirmware or software, the present invention can be implemented in theform of a variety of formats, for example, modules, procedures,functions, etc. The software codes may be stored in a memory to bedriven by a processor. The memory is located inside or outside of theprocessor, so that it can communicate with the aforementioned processorvia a variety of well-known parts.

The detailed description of the exemplary embodiments of the presentinvention has been given to enable those skilled in the art to implementand practice the invention. Although the invention has been describedwith reference to the exemplary embodiments, those skilled in the artwill appreciate that various modifications and variations can be made inthe present invention without departing from the spirit or scope of theinvention described in the appended claims. For example, those skilledin the art may use each construction described in the above embodimentsin combination with each other. Accordingly, the invention should not belimited to the specific embodiments described herein, but should beaccorded the broadest scope consistent with the principles and novelfeatures disclosed herein.

Those skilled in the art will appreciate that the present invention maybe carried out in other specific ways than those set forth hereinwithout departing from the spirit and essential characteristics of thepresent invention. The above exemplary embodiments are therefore to beconstrued in all aspects as illustrative and not restrictive. The scopeof the invention should be determined by the appended claims and theirlegal equivalents, not by the above description, and all changes comingwithin the meaning and equivalency range of the appended claims areintended to be embraced therein. Also, it will be obvious to thoseskilled in the art that claims that are not explicitly cited in theappended claims may be presented in combination as an exemplaryembodiment of the present invention or included as a new claim bysubsequent amendment after the application is filed.

MODE FOR INVENTION

Various embodiments have been described in the best mode for carryingout the invention.

INDUSTRIAL APPLICABILITY

The embodiments of the present invention can be applied to a variety ofmobile communication systems.

1. A method for acquiring system information by a user equipment (UE) ina wireless communication system, the method comprising: performing ahandover from a first cell to a second cell; and acquiring systeminformation of the second cell, wherein a method for acquiring thesystem information of the second cell is determined according to CRE(Cell Range Extension)-related capability of the UE.
 2. The methodaccording to claim 1, wherein: if the UE has PBCH (Physical BroadcastChannel) cancellation capability, the UE acquires system information ofthe second cell through a PBCH of the second cell.
 3. The methodaccording to claim 2, wherein the UE performs PBCH cancellation of thefirst cell when acquiring the PBCH of the second cell.
 4. The methodaccording to claim 1, wherein: if the UE does not have the PBCHcancellation capability, the UE acquires system information of thesecond cell from the first cell.
 5. The method according to claim 4,wherein the system information of the second cell is transferred to theUE in a step of performing the handover.
 6. The method according toclaim 1, further comprising: if a predetermined condition is satisfiedafter completion of the handover, transmitting an error report to thesecond cell.
 7. The method according to claim 6, wherein thepredetermined condition is that the UE fails to successively receivesignals from the second cell.
 8. The method according to claim 6,further comprising: receiving at least one of changed system informationand specific information indicating no change of the system information,as a response to the error report.
 9. The method according to claim 1,further comprising: if the UE is located within a predetermined distanceof the second cell after completion of the handover, transmitting arequest of system information to the second cell.
 10. The methodaccording to claim 1, further comprising: receiving system informationmasked with C-RNTI of the UE, as a response to the system informationrequest.
 11. The method according to claim 9, wherein the systeminformation request is any one of an MIB (Master Information Block)request, an SIB (System Information Block) request, and a pagingrequest, which are updated information of the second cell.
 12. Themethod according to claim 1, wherein the first cell is a macro eNB, anda second cell is a pico eNB.
 13. The method according to claim 1,wherein the CRE-related capability information includes at least one ofreference signal (RS) cancellation capability and synchronization signalcancellation capability.
 14. A user equipment (UE) apparatus for use ina wireless communication system comprising: a reception (Rx) module; anda processor, wherein the processor performs a handover from a first cellto a second cell and acquires system information of the second cell,wherein a method for acquiring the system information of the second cellis determined according to CRE (Cell Range Extension)-related capabilityof the UE.